How can a rider that's riding it for the first time possibly be anything other than unprepared for that?

One of the people in a coma is a kid (age unspecified), so if there's kids being injured and 70+ other cases of injury identified in their records, chances are it definitely is rough enough to cause issues even in people without a "previous condition" (previous condition: intact blood vessels in brain?).

If the park knew of previous grievous injuries and did nothing to remediate future ones, they're likely liable - especially if they did not publicise the known risks clearly to future riders.